Over the past two decades, several techniques have been developed applicable to continuous direct reading monitoring of airborne particulate mass concentration. The relevant sensing techniques applicable to particulate monitoring are (a) beta radiation attenuation, (b) resonant frequency decrement, and (c) nephelometry.
1. Beta Radiation Attenuation
This technique has been incorporated in numerous experimental as well as commercial instruments in the U.S. and abroad. These devices typically fall into the following subcategories:
(a) filter tape collectors PA1 (b) filter cartridge collectors,and PA1 (c) single-stage nozzle/plate impactors.
Each of the above radiation attenuation devices include a mass sensing stage, which is either separate from or combined with a particle collection stage. The mass sensing stage further includes a beta particle radiation source, typically carbon-14 or krypton-85, and a beta particle detector, typically a Geiger-Muller detector, plastic scintillator, or ionization chamber, which are placed on opposite sides of the collection substrate.
The attenuation of Beta electrons results from their interaction with the orbital electrons of the atoms in the path of the beta particle. The details of this process as it relates to aerosol particle monitoring may be found in the literature (Lilienfeld, P., Design and Operation of Dust Measuring Instrumentation Based on the Beta-Attenuation Method, Staub, Vol. 35, p. 458, 1975).
Specific characteristics of beta attenuation devices are as follows:
(a) mass measurement sensitivity increases when decreasing the collection area, when decreasing the emitted beta energy, and when increasing the total registered beta count;
(b) mass measurement errors arise from variations of the air density within the sensing gap during the period between the initial and final beta count cycles used to determine the mass increment. Further errors arise from sensitivity changes of the beta detector (e.g., temperature-related drift), particle collection inhomogeneities on the filter (or other substrate) area, the presence of elements whose ratio of atomic number to atomic mass deviates significantly from that of the elements used in the reference calibration, and changes of geometry between initial and final beta counting cycles (e.g., filter tape transport inaccuracies); and
(c) additional factors contributing to measurement inaccuracies include intrinsic radioactivity of the collected sample, detector coincidence counting losses, as well as several other factors.
An important, albeit somewhat subjective, disadvantage of the beta attenuation technique is the negative perception associated with the use of a radioactive source which, although usually of low energy and low total activity (e.g., E.sub.max =150 KeV for C-14 and 100 microcuries), nevertheless represents a perceived potential for radioactive contamination.
Lastly, the inherent sensitivity of the beta technique is marginal for short measurement periods (e.g., 30 minutes as with the present invention) unless extreme collection area concentrations are achieved, i.e., by means of nozzle/plate impaction. However, impaction entails additional particle collection drawbacks which would limit the device's usefulness in the context of the present invention.
2. Resonant Frequency Decrement
Any mechanical system including a mass in combination with a mechanical energy storage device (e.g., a spring) will tend to oscillate harmonically at its so-called natural resonance frequency, which is proportional to the square root of the ratio of the system stiffness constant and its mass. As this mass increases (e.g., by particle collection), the system resonant frequency decreases. This frequency decrement is a measure of the collected particle mass. There are a number of embodiments of this technique developed since the 1960's. The most important of these techniques are:
(a) oscillating wires or ribbons used as particle impaction surfaces (see Gast, T., Acoustical Feedback as Aid in the Determination of Dust Concentrations by Means of an Oscillating Ribbon, Staub, Vol., 30, p. 1, 1970 and Gast, T. and Bahner, H., Fortschritte bei der Messung yon Feststoffmengen mit Hilfe eines Schwingenden Bandes, Staub, Vol. 39, p. 109, 1979);
(b) quartz crystal piezo-balance (Olin, T. G., Sem, G. J. and Christenson, D. L., Piezo-Electrostatic Aerosol Mass Concentration Monitor, AIHR Jour., Vol. 209, 1971);
(c) tapered element oscillating mass monitor or TEOM (Patashnick, H. and Rupprecht, G., A New Real-Time Aerosol Mass Monitoring Instrument: the TEOM, Proceedings: Advances in Particle Sampling and Measurement, EPA-600/9-80-004, p. 264, 1980); and
(d) oscillating filter tape monitor or MESA (Poss, G., Krann, U. and Solmos, A., A New Instrument for Online Dust Monitoring, Aerosols: Formation and Reactivity, 2nd Int. Aerosol Conf. Berlin, p. 782, Pergamon Press, 1986).
Of the above techniques, methods (b) and (c) have received relatively broad use. The quartz crystal micro-balance method combines either electrostatic precipitation (TSI, Inc.) to collect the particles on the surface of a quartz crystal (which is part of a resonant electronic circuit), or jet-to-plate impaction (e.g., California Measurements cascade piezo-balance impactor).
The principal advantage of the resonant mass monitoring technique, however, is that it represents the closest approximation to a reference gravimetric method, which cannot be used in many environments because gravimetric methods are not compatible with automated, continuous, indicating/recording of dust measurements. The resonant mass monitoring technique is, moreover, a direct mass sensing approach, as opposed to the other continuous sensing methods. It can be also considered as an absolute method in that the measured frequency decrement is predictably related to the increment of the collected particulate mass.
The principal disadvantages of the resonant technique are as follows:
(a) its inherent dependence on collected mass precludes discrimination against interference by liquid particles, especially water in the case of coal mines (a shortcoming shared with the beta attenuation technique);
(b) sensitivity to varying relative humidity, which is principally associated with filter material hygroscopicity; and
(c) second or higher order effects affecting the stiffness constant of the resonating system.
These characteristic problems of resonant sensors will be addressed in detail in the subsequent section describing the central method proposed in the present invention: the resonant filter membrane mass monitor.
3. Nephelometry
Nephelometry is based on the measurement of the intensity of the light scattered by an ensemble of airborne particles, as opposed to single particle counting by light scattering. For an aerosol with a fixed size distribution of particles with invariant density and index of refraction, the intensity of light scattered within a given sensing configuration is directly proportional to the mass concentration of that aerosol (in the absence of multiple or of dependent scattering conditions which usually do not apply to the case under scrutiny). Although the effects of the particle size and index of refraction (on the relationship between light scattering vs. mass concentration) can be minimized for a given range of conditions, excessive deviations can not be avoided considering the entire range of aerosol parameters that may be encountered.
The principal advantages of nephelometry in this context are: a) superior sensitivity allowing true real-time measurements, b) relative simplicity of the sensing stage, c) compatible with miniaturization, ruggedization and long term maintenance free operation, and d) inherently independent of flow rate (except when used in conjunction with an active sampling system incorporating size selective inlet elements) allowing passive sampling which, in turn, minimizes power requirements and, consequently, overall weight.
It is important, however, to clarify that the above stated particle size and index of refraction limitations apply to monoparametric (i.e., basic intensity) nephelometry which in the present application is labelled as "dumb" nephelometry. By contrast, "smart" nephelometry, i.e. multiparameter nephelometry, can provide a far more complete characterization of an aerosol, virtually eliminating particle size and refractive index effects on mass measurements, and leaving only particle density as a "loose" parameter.
"Smart" nephelometry consists of the detection of light scattering as a function of one or more parameters that depend on particle size and index of refraction and--as no other real-time method can accomplish--particle shape. These parameters are: a) the phase function or scattering intensity as a function of scattering angle, b) wavelength dependence of the scattering and, in combination with the former, c) polarization/depolarization characterization. "Smart" nephelometry is especially useful as a means to detect and quantify the proportion of liquid water aerosol.
4. Summary of Background Art
Of the three salient techniques discussed in the background section, only the resonant oscillating method is strictly dependent upon mass and is the basis for the present invention. Heretofore, however, the resonant mass technique has suffered from a significant disadvantage: the need for either routine manual replacement of the particle collection medium (TEOM) or the requirement for elaborate and cumbersome collection surface restoration schemes (automated quartz crystal microbalance). In addition, devices such as the TEOM are susceptible to positional and vibrational effects which are of little or no consequence in well controlled stationary fixed point applications, but nevertheless become seriously limiting factors where these devices are to operate reliably within severe environments, e.g. blasting sites, coal mines, etc.
Other idiosyncrasies of TEOM-like devices are: the requirement for an inlet "chamber" from which the particle laden flow passes to the filter cartridge, i.e. this filter cannot be coupled directly to the system inlet (e.g. the cyclone exhaust port) because it must be allowed to oscillate freely, attached only to the tapered element. This inlet chamber has the potential for creating particle wall losses due to turbulent impaction, sedimentation and/or electrostatic forces.
Quartz crystal piezobalances exhibit other serious practical limitations that have constrained their application to largely laboratory uses. Principal among these problems are: exceedingly small total accumulated mass capacity of the quartz crystal (i.e. of the order of 100 .mu.g) requiring very frequent crystal cleaning; and problems with particle adherence (vibrational coupling) to crystal, especially for particles larger than about 3 .mu.m and chain aggregates (such as combustion aerosols).
Other mechanical resonance schemes (some of which were mentioned in the background section) have proven impractical or insensitive: the taut wire or ribbon with transverse oscillation through excitation by capacitive means and particle collection by impaction investigated in the 1960's; the oscillating arm/pleated filter cartridge-spring-restored scheme developed at the General Electric Co. around 1970; and the more recent MESA system of Posh et al., incorporating an oscillating filter tape length with piezo-electric longitudinal excitation, applied to source emission monitoring (i.e. at concentrations of 10 to 1000 mg/m.sup.3).
The above brief review demonstrates a long felt need for an improved sensing system and method which will accurately measure and monitor airborne particle concentrations.
It is an object of the present invention to provide a system and method for measuring and monitoring airborne particle concentrations.
Another object of the present invention is to provide a system and method for measuring and monitoring airborne particle concentrations in dusty environments which are potentially harmful to human health.
The foregoing specific objects and advantages of the present invention are illustrative of those which can be achieved by the present invention and are not intended to be exhaustive or limiting of the possible advantages which can be realized. Thus, these and other objects and advantages of the invention will be apparent from the description herein or can be learned from practicing the invention, both as embodied herein or as modified in view of any variations which may be apparent to those skilled in the art. Accordingly, the present invention resides in the novel parts, constructions, arrangements, combinations and improvements herein shown and described.